Siloxanes represent a widely used class of organic silicon compounds. They are oligomer or polymer compounds, in which adjacent silicon atoms are bridged by oxygen atoms and which typically have the general formula R13Si—O—[SiR12O]n—SiR13, in which R1 independently represents hydrogen or alkyl. A typical representative of such siloxanes is hexamethyldisiloxane (HMDS).
Their extensive application possibilities and lack of toxicity are leading to an increasing widespread use of siloxanes. Siloxanes are used, for example, as components of dyes, paints, cosmetic articles, adhesives, impregnating agents and insulating agents and construction materials as well as auxiliary agents in the processing of plastics, and siloxane concentrations of a few ppb up to in the ppm range are often obtained because of the high vapor pressure of siloxanes in ambient atmosphere. In addition, siloxanes occur in said concentrations, for example, also as associated gas in biogas, sewage gas and landfill gas.
If siloxanes are involved in combustion processes, then silicon dioxide forms in this connection. The conversion of the siloxanes into silicon dioxide and the subsequent deposit thereof as solid can cause serious damage to technical equipment and measuring instruments.
For example, the presence of siloxanes in biogas and digester gas, which is used in the generation of energy by means of combustion, by means of the conversion to silicon dioxide and deposits resulting therefrom leads to damage to the combustion machines. For that reason, biogas has to be freed from siloxanes by means of a complicated treatment (M. Ajhar et al., Bioresource Technology 2010, 101, 2913-2923).
The forming of silicon dioxide may also lead to safety-relevant damage in gas sensors according to the heat tone principle or in so-called semiconductor sensors. If siloxanes are present in the gas atmosphere to be analyzed (e.g., in a plastics-processing factory or in a treatment plant), these are converted to silicon dioxide on the gas-sensitive layer of heat tone sensors and semiconductor sensors and the silicon dioxide is deposited on the gas-sensitive layer. The access of the analyte to the gas-sensitive layer is consequently hindered and the sensitivity of the gas sensor is reduced (so-called sensor toxification).
In case of a high siloxane load of the gas atmosphere to be analyzed, a reduction in the sensitivity of the sensor to half or even less can occur within a few hours because of this sensor toxification. The sensor can then no longer warn against an explosive atmosphere, or only to a limited extent. This is very precarious for safety reasons directly because of the usually long calibration interval of up to 3 months in some cases. A maintenance-intensive functional test must consequently be carried out more frequently in case of gas atmospheres loaded with siloxanes. Measuring sensors according to the heat tone principle or semiconductor principle must even be dispensed with entirely in case of very high loads and be switched to measuring principles (e.g., IR absorption) which are more cost-intensive and have drawbacks.
Therefore, various ways to remove siloxanes from gases have been pursued. Thus, the removal of siloxanes by means of condensation at a temperature of −25° C. is frequently carried out, for example, in the treatment of biogas. This method is, however, technically complicated and requires a great deal of energy for cooling to the necessary low temperature.
In some cases, adsorbers based on silica gel, highly disperse aluminum oxide, zeolites or activated carbons are also used in the treatment of biogas as well as in gas sensors. However, these materials bind siloxanes only weakly and unselectively. Very high absorber capacities must hence be provided to ensure that siloxanes are absorbed to a sufficient extent. Because of the low selectivity, this leads to other gas components also being absorbed, which leads to the loss of industrially exploitable gas components in industrial applications or the adulteration of the gas sample in analytical applications.
EP 0 094 863 A1 and WO 00/43765 A1 describe sensors for combustible gases according to the heat tone principle, in which the catalytically active sensor element is embedded in a porous absorbent material such as especially a zeolite material, as a result of which the sensor shall be insensitive to sensor toxins such as siloxanes. Nevertheless, it has been shown that the absorber material in such gas sensors reduces the diffusion rate of the gas to be analyzed, which leads to a reduced sensitivity, to a greatly prolonged response time and thus to a marked limitation of the detectable gases. As a rule, the response time and sensitivity of such sensors are only sufficient for methane and hydrogen, while higher alkanes such as propane and butane as well as other combustible substances can no longer be detected with certainty.